One of the basic components in microwave circuits is the differential phase shifter, most frequently used in the design of 45 degree, 90 degree, and 180 degree phase shifters. Differential phase shifters are useful in phased array antennas, distribution networks, balanced amplifiers, two-diode image rejection mixers, two-diode pseudo-image enhancement mixers and other circuit functions.
Differential phase shifters can be conveniently constructed 5 using parallel-coupled transmission lines of equal length operating in the transverse electromagnetic mode (TEM mode), connected at one end, as first described by B. M. Schiffman in the paper entitled "A New Class of Broadband Microwave 90 Degree Phase Shifters," IRE Transactions MTT, Apr., 1958, pages 232-237, herein incorporated by reference. The unconnected ends of the transmission lines serve as the input and output of a two-terminal-pair network.
Realization of such phase shifters in stripline and coaxial transmission line implementations has yielded excellent wideband performance, including high return loss, low insertion loss, and small phase shift deviation versus frequency. Attempts to make similar Schiffman phase shifters in microstrip transmission lines has typically resulted in poor performance. The primary reason for this appears to occur as a result of the difference in phase velocity between odd and even modes in a coupled microstrip transmission line.
One method of compensating for poor performance in microstrip Schiffman phase shifters is to use stepped coupled line structure as described in "Broadband Matching of Microstrip Differential Phase Shifters" by B. Schiek, J. Kohler, and W. Schilz, 1976 European Microwave Conference Proceedings, pages 647-651. In the stepped coupled line structure approach, the coupled region is divided into two sections of equal length with different even and odd mode characteristic impedances.
While substantially improved results can be obtained with the stepped coupled line structure approach, disadvantages include the following:
a) The electrical length of each section is not an arithmetic mean of the even and odd mode microstrip parameters; and PA1 b) The introduction o f step discontinuities in realizing different coupled sections degrades performance.
Thus, what is needed is a Schiffman differential phase shifter suitable for MMIC implementation that provides ideal performance over wide bandwidth.